In a high-speed and long-distance communication transmission, it is difficult to meet relevant requirements by intensity modulation. At present, during the long-distance transmission in the optical fiber communication field, the phase modulation technology is required. The present photoelectric phase modulation is generally implemented by Mach-Zahnder (MZ) modulators (hereinafter referred to as MZ modulators). However, heat generated during the running of the MZ modulators, the change of environmental temperature and ageing resulted from long-term running will influence the intensity of an electric field, and it is likely to change the characteristics of photoelectric modulators, so that the ideal control point of the modulators drifts from a preset point. As a result of drifting of the ideal control point, the curve amplitude and central position of the modulated optical signal change so that the optical eye diagram is degraded. When a serious drifting is caused, an MZ modulator will show strong nonlinearity. As a result, the maximum dynamic range of the optical communication connection will be reduced, the performance of the whole system will be degraded, and even the original information cannot be restored from the received optical signals if serious. Therefore, the stable control to working points of the light modulators has to be achieved. Shown in FIG. 1 is a schematic diagram of an existing working points of an modulator.
Currently a common controlling method for MZ modulator offset point is as follows: a low-frequency square signal is added into a modulation signal of the MZ, then this signal is separated from the output modulation signal, and finally the stability of the offset point is controlled through phase-locked amplification, as shown in FIG. 2. The controlling circuits become very complex due to the full-hardware control; meanwhile, the accuracy of the whole controlling loop is limited by the temperature stability of circuit devices.